1. Field of the Invention
The present invention relates to a temperature sensor.
2. Description of Related Art
It is known to detect the temperature of exhaust gas of an automobile using a temperature sensor having a thermo-sensitive element such as a thermistor element whose resistance varies depending on the ambient temperature. Such temperature sensors include ones that have a thermo-sensitive element, electrode wires connected to the thermo-sensitive element, signal wires connected to the electrode wires, and a sheath pin accommodating the signal wires in a state where the signal wires are exposed from the sheath pin at their front ends. Commonly, Pt is used as a material of the electrode wire, and SUS310S is used as a material of the signal wire.
Meanwhile, it is proposed to dispose an intermediate layer between the electrode wire and the signal wire. For example, Japanese Patent Application Laid-open No. 2009-294107 discloses a temperature sensor having the structure in which an intermediate member having a thermal expansion coefficient in between the thermal expansion coefficient of an electrode wire and the thermal expansion coefficient of a signal wire is disposed in an overlap portion between the electrode wire and the signal wire, and the intermediate member and its vicinity are laser-welded to form an intermediate layer.
However, the above temperature sensor is not suitable for use of detecting a wide temperature range, for example, a temperature range from around −40° C. to +1000° C. because of the following reason. In the temperature sensor using Pt as a material of the electrode wire and SUS310S as a material of the signal wire, the difference between the thermal expansion coefficient of the electrode wire and the thermal expansion coefficient of the signal wire is large. Accordingly, the thermal stress in a junction part between the electrode wire and the signal wire increases with the increase of the temperature difference between the hot state and cold state in use. Hence, the electrode wire which is weaker in material strength than that of the signal wire is likely to break. This problem becomes conspicuous particularly in an environment where the temperature sensor is repeatedly subjected to a hot and cold cycle.
Incidentally, in the temperature sensor described in the above patent document, the intermediate layer is formed by disposing the intermediate member having a thermal expansion coefficient in between the thermal expansion coefficient of the electrode wire and the thermal expansion coefficient of the signal wire. However, it is difficult to completely prevent breakage of the electrode wire by using such a structure, because the intermediate layer and the signal wire are likely to be oxidized.